If you want to enable @-expressions in a module that is written in,
say, `scheme/base', the old way was to start the file like this:
#reader scribble/reader
#lang scheme/base
The new way is like this:
#lang at-exp scheme/base
In general, `at-exp' is a kind of language constructor that installs
"@" support (through the readtable) before chaining to a base language.
Eventually, DrScheme and other tools will work better on files that
start "#lang". The old "#reader" way works only because an S-expression
reader happens to be active before "#lang".